Much work has been done to make internally plasticized and internally stabilized polyvinyl chloride (PVC) by employing copolymerizable lubricants and stabilizers. Copolymers of vinyl halides such as vinyl chloride and vinyl stearate were extensively studied with the aim of making internally plasticized PVC via the incorporation of vinyl stearate at levels of about 10 to 50% by weight. Vinyl chloride/vinyl stearate copolymers with very low levels of vinyl stearate were not reported and were of no commercial interest, because at such low levels of vinyl stearate the copolymers' mechanical properties were thought to be not much different than those of the PVC homopolymer.
Copolymers of vinyl chloride with vinyl stearate are well established in the literature. The mechanical properties of suspension copolymers of vinyl chloride with vinyl esters of long chain fatty acids, such as vinyl stearate, are discussed by W. S. Port, E. F. Jordan, Jr., W. E. Palm, L. P. Witnauer, J. E. Hansen and D. Swern; Industrial and Engineering Chemistry, Vol. 47, pp. 472-480 (1955). Homopolymers, copolymers and terpolymers of allyl stearate/vinyl stearate/vinyl chloride are discussed by E. F. Jordan, Jr., G. R. Riser, B. Artymshyn, S. Smith, Jr., and A. N. Wrigley; Journal of Polymer Science: Polymer Chemistry Edition, Vol. 11, pp. 1475-1504 (1973). U.S. Pat. No. 2,993,034 discloses copolymers of vinyl chloride and vinyl epoxy stearate.
The demand for specialty polyvinyl chloride resin for use in custom injection molding (CIM) applications initiated research for PVC resins having high melt flow and low molecular weight, without any corresponding adverse effects on heat stability, heat distortion and related properties. The use of low levels of vinyl esters of fatty acids as a copolymer for PVC substantially improves the melt flow properties, but there is a further need to reduce the molecular weight of the polymer. The molecular weight reduction is accomplished through the practice of this invention by the use of a mercapto compound as a chain transfer agent.
One of the most efficient chain transfer agents in the family of mercaptans is 2-mercaptoethanol. It is more effective than other conventional chlorinated chain transfer agents, such as trichloroethylene. U.S. Pat. No. 4,189,552, which is herein incorporated by reference, discloses the use of 2-mercaptoethanol in the suspension polymerization of vinyl chloride with 2-mercaptoethanol levels of 0.001 to 0.50 part per 100 parts of monomer. The '552 patent teaches that the introduction of the mercapto compounds into the reaction medium at a time when the monomer conversion is lower than 1% produces adverse effects on particle characteristics. Because of the detrimental effects that mercapto compounds have on colloidal stability, the '552 patent instructs that the mercapto compounds should be introduced by divided addition during the polymerization. In essence, 2-mercaptoethanol at levels greater than 0.03 part per 100 parts monomer, charged before the onset of the polymerization, is likely to cause an unacceptable coarse or solid charge.
It is a desirable goal to be able to use highly efficient mercaptan chain transfer agents, such as 2-mercaptoethanol, at high levels and without resorting to step wise addition while maintaining the colloidal stability of an aqueous polymerization reaction.